Ferroelectrics have received considerable interest due to their applicability for a number of potential technologies. In bulk form they have high permitivities (dielectric constant), have large electromechanical coupling coefficients, exhibit pyroelectric behavior, and have electro-optic effects. In thin-film form they have potential applications for optical waveguides, optical modulators and shutters, optical displays and memories, piezoelectric transducers, decoupling capacitors, pyroelectric detectors and ferroelectric memories. Of particular interest is the integration of passive optical devices (e.g. waveguides) with active electro-optic devices (e.g. modulators) and conventional microelectronic devices for optoelectronic integrated circuits (OEICs).
In recent years, researchers have fabricated various thin-film waveguides for integrated optical devices. Titanium-diffused lithium niobate waveguides have been studied; however, problems with optical damage and weak electro-optic effects have limited their practical use, as noted in "A New Waveguide Switch/Modulator for Integrated Optics", by W. E. Martin, Appl. Phys. Vol. 26 (1975), p. 562; "Optical Channel Waveguide Switch and Coupler Using Total Internal Reflection", by C. S. Tsai et al., IEEE J. Quantum Electron., Vol. QE-14, (1978), p. 513; and "Optical Damage Resistance of Lithium Niobate Waveguides", by R. L. Holman et al. Opt. Eng., Vol. 21 (1982), p. 1025. Fabrication of thin-film lanthanum-modified-lead-zirconate-titanate (PLZT) ferroelectric ceramics have been proposed to overcome the limitations exhibited by lithium niobate. Such thin-films have been formed by rf planar magnetron sputtering of a powder target onto a sapphire substrate, see, for example, "Electro-optic Effects of (Pb,La)(Br,Ti)O.sub.3 Thin Films Prepared by rf Planar Magnetron Sputtering", by H. Adachi et al., Appl. Phys. Lett., Vol. 42 (983), p. 867; and "PLZT Thin-film Waveguides", by T. Kawaguchi et al., Appl. Optics., Vol. 23 (1984), pp. 2187-2191. Ridge-type channel waveguides have been fabricated using ion-beam etching techniques. These techniques are necessary since there is no suitable conventional etchant for PLZT films, and high temperature processes can result in out-diffusion of lead from the thin film, as noted in above reference by Kawaguchi. Ion beam etching techniques have a number of limitations, however: (1) etching selectivity between the thin-film and the photoresist mask is poor (typically 1.2:1), (2) etch rates are low (typically 13 nm/min), (3) typical etch non-uniformities are large (.+-.10%), and (4) there is limited control of the resulting surface quality. These limitations inhibit low cost, high yield fabrication of integrated devices and also affect device performance. For example, optical propagation losses from surface scattering from roughened top surface or sidewalls in waveguides must be controlled to obtain useful optical structures, note the article by D. Marcuse, "Mode Conversion Caused by Surface Imperfection of a Dielectric Slab Waveguide", Bell Syst. Tech. J., Vol. 48 (1969), p. 3187.
Recent advances in polymeric solution-gelation (sol-gel) processing of ferroelectric ceramics offers new hope for integrated waveguides. Research in sol-gel processing has addressed the requirements of fabricating ferroelectric ceramics for electronic applications such as high permitivity dielectrics, non-volatile memory elements or optical image storage such as that shown in the articles "Sol-Gel-Derived PbTiO.sub.3 " by Blum et al., J. Mater. Sci., Vol. 20 (1985), pp. 4479-4483; "PbTiO.sub.3 Films from Metalloorganic Precursors" by R. W. Vest et al., IEEE Trans. UFFC, Vol. 35 (1988), pp. 711-717; "Integrated Sol-Gel PZT Thin-Films on Pt, Si, and GaAs for Non-Volatile Memory Applications" by S. K. Dey et al., Ferroelectrics, Vol. 108 (1990), pp. 37-46; and "Thin-Film Ferroelectrics of PZT by Sol-Gel Processing" by S. K. Dey et al., IEEE Trans. UFFC, Vol. 35 (1988), pp. 80-81. The sol-gel procedure involves the synthesizing of precursor complexes by vacuum distillation of metalloorganic compounds. The precursor complexes are subsequently hydrolyzed and condensed to form stable polymeric solutions which can be spin cast on substrates using conventional techniques. The resulting thin-film precursors undergo a low temperature annealing to volatilize organics, and are annealed at higher temperatures to crystallize and densify the film, see the first cited by Dey et al. article above, and pending USPTO application Ser. No. 07/709,671 by S. D. Russell et al. "Method of Laser Processing Ferroelectric Materials". Extensions of the existing chemistry can be envisioned by one skilled in the art to include the addition of a lanthanum-based compound in the formation of a PLZT sol-gel precursor film or other useful optical ceramics.
Thus, in accordance with this inventive concept a need has been recognized for a method using the sol-gel process described above in operative association with a predetermined sequence of process steps for fabricating a "waveguide mold" which results in an improved conformal processing method of producing ferroelectic ceramic waveguides that is integratable with conventional electronic and optoelectronic devices.